Recursos técnicos

Corrosion resisting alloys (CRAs), as the name suggest are alloys that have been designed to and are able to resist degradation in environment or media where they are used. It is worth defining corrosion then as the degradation of a material or alloy as a result of its interaction with its specific environment.

The Corrosion Phenomenon – Inevitable and Costly.

In the world of materials and alloys, corrosion phenomenon is inevitable and costly.

It is a naturally occurring, thermodynamic change process. As a result, owner/operators need to apply a life cycle cost benefit analysis approach when deciding on material selection investments in their various projects. Unlike other phenomena, corrosion as well as other integrity issues cannot be suspended, deferred and halted in recessionary times; they just simply go on as they are no respecter of economic cycles.The good news is that corrosion can be controlled and slowed down dramatically; say to rates less than 0.1mm/year. This is achieved by right materials selection, design, coatings inhibition and cathodic protection as applicable in various industries.

How do Corrosion Resistant Alloys protect against Corrosion?

CRAs rely on the existence and stability of a passive film on its surface to act as a protective barrier to deleterious interactions between the environment and component.

The passive film is usually chromium oxide for most stainless steels, nickel alloys, cobalt based alloys and titanium oxide in titanium alloys. This passive film is extremely thin – in order of nanometres (1 – 5 nm), very adherent and invisible to the naked eye. They readily rapidly form and are very stable in the presence of oxygen in air, water and other aqueous solutions (acids, bases, process fluids, salts, etc.). The presence of chromium, alongside other alloying elements such as molybdenum, nitrogen, tungsten and copper in CRAs does improve corrosion resistance of alloys in diverse environment.

Due to this passive film presence, general uniform corrosion is seldom an issue, what is more prevalent is localised corrosion inform of pitting corrosion, crevice corrosion and stress corrosion cracking, leading to other failure mechanisms such as galvanic corrosion and microbial induced corrosion. This will thus manifest initially as a pit or crevice, then crack, fracture and eventual collapse.
The key drivers or influencers of this type of corrosion are oxygen, halides (especially chlorides), temperature, and flowrate, presence of impurities or contamination and microbial activity from organic matter.

How are CRA’s Ranked?

CRAs are ranked for performance in process, chemical, oil and gas industry by a unique number called the Pitting resistance equivalent number PREn, which takes into account the effect of specific elements such as chromium, molybdenum, nitrogen, and sometimes tungsten; as they contribute to localised corrosion resistance of individual alloys.

718 is a precipitation hardened nickel – chromium alloy. It combines high strength in the aged condition with good corrosion resistance and weldability. Commonly used in oil and gas exploration.

N07718

Ti: 0.9,

Cb+Ta: 5.0

Alloy 400

63

–

–

2.5

Cu: Rem,

Alloy 400 from NeoNickel is used for its superb combination of corrosion resistance, weldability and ductility. The corrosion resistance in seawater is especially good under high velocity conditions.

N04400

C: 0.30,

Mn: 2.00,

Si: 0.50

65

C 276

Rem

15

15

7 max

C: 0.02 max

C276 is a solid-solution strengthened, nickel-molybdenum-chromium alloy with a small amount of tungsten, which exhibits excellent corrosion resistance in an assortment of harsh environments.

N10276

Mn: 1 max

W: 4.5 max

65

C 22

52

22

13

6

C: 0.015 max,

Alloy 22 is a fully austenitic, nickel-chromium-molybdenum-tungsten alloy with high corrosion resistance compared to other nickel-chromium-molybdenum alloys. The high chromium content provides good resistance to oxidizing conditions while the molybdenum and tungsten content give good resistance to reducing media. This alloy provides exceptional resistance to pitting, crevice corrosion, general corrosion and stress corrosion cracking.

N06022

Co: 2.5 max,

Mn: 0.5 max,

W: 3.5 max

Titanium alloys

Ti

O2

C

Fe

Others

Grade 2 Unalloyed R50400

Rem

0.25 max

0.10 max

0.3

H: 0.015 max

Titanium CP3 Grade 2 possesses excellent welding properties, excellent resistance to oxidation and corrosion, good strength and superb cold forming properties making it the alloy of choice for industries globally.

max

N: 0.03 max

Grade 5

89.4

0.16

0.10 max

0.15

Al: 6.0,

Ti 6Al-4V (Grade 5) is the most widely used of all the alpha-beta titanium alloys. It is typically used in the annealed condition, at service temperatures through 399°C. It offers high strength and excellent general corrosion resistance.

Ti 6Al-4V

V: 4.0

R56400

*PREn is calculated based on actual ladle analysis chemistry thus may vary

Common Aqueous Environments and Suggested Alloys

Corrosion in component parts, equipment or assets can be expensive, especially in difficult to reach environment or high shutdown cost application.

More unforgiving is localised corrosion as it occurs very fast and the damage is usually done, then a realisation. This suggests a real need for expert advice to minimize risk, damage and impact to environment, costs and above all health and safety concerns.

The table below is a performance guidance showing relative resistance of some of the most common aqueous application environment;

Wet corrosion

Environment

Not suggested

Good

Better

Best

Performance guide

Chlorides

304L.

Alloy 20, 316L,

400(a), 2205, 317L.

AL-6XN, 625, C-276,

(Pitting, crevice corrosion)

LDX 2101, 600.

Titanium, C22,

Alloy 59, ZERON 100.

Chloride stress corrosion cracking

304L, 316L.

LDX 2101, 904L, 2205,

AL-6XN, Alloy 20,

400, 600, 625,

317L

ZERON 100.

Alloy 59, C-276, C22.

Hydrochloric acid

Titanium(b), 600, Alloy 20, 2205,

200(a), 400(a),625,

C22, C-276, 686.

Zirconium(a),

LDX 2101, 317L.

ZERON 100

HASTELLOY® B2, B3 (a),

Tantalum, Titanium (b).

Hydrofluoric acid

200, 600, 2205, etc.

C-276, C22,

400(a), Silver(a)

Gold, Platinum.

Alloy 59,

400 (N purged).

Sulphuric acid

Titanium, 600.

316L, 317L, LDX2101,2205

AL-6XN, 625

Alloy 20, C-276, Tantalum,

ZERON 100.

Phosphoric acid

200, 400, 316L, 317L.

904L, 2205

AL-6XN, Alloy 20,

G-30, 625.

(commercial)

ZERON 100.

Nitric acid

904L, AL-6XN, 200,

304L, Alloy 20, 2205, ZERON 100

625

Zirconium, Tantalum.

400, 600.

Caustic

304L, 316L, 317L,

Alloy 20, 2205,

600, 625, 400, 686, C22, C-276.

200(a).

Tantalum.

LDX2101,

ZERON 100.

(a)Presence of oxygen or oxidizing salts may greatly increase corrosion. (b)Titanium has excellent resistance to hydrochloric acid containing oxidizers such asFeCl3, HNO3, etc. However, titanium has very poor resistance to pure, reducing, HCl. This chart is intended as guidance for what alloys might be tested in a given environment. It must NOT be used as the major basis for alloy selection, or as a substitute for competent corrosion engineering work.

Hastelloy® is a registered trademark of Haynes International

You could contact NeoNickel for support and as your CRA vendor in Europe.

Our metallurgist and corrosion engineers have an in-depth knowledge and vast experience across industries in corrosion applications and are happy to support clients regarding advice with material related issues, alloy performance and material failure analysis.